1. Field of the Invention
The present invention relates to a technique for controlling exposure and charge accumulation using an electronic shutter function that controls the charge accumulation time in an image sensing element and a mechanical shutter function for shielding the light receiving surface of the image sensing element with shutter blades.
2. Description of the Related Art
There are image sensing apparatuses such as digital cameras provided with a progressive scan CCD sensor (hereinafter, “PS-CCD”) as an image sensing element. If a PS-CCD sensor is used as the image sensing element, the image sensing apparatus is able to control the exposure for image data to be obtained from the image sensing element by controlling the charge accumulation period from when charge is discharged until when accumulated charge is transferred. While the image sensing apparatus is able to regulate the charge accumulation period even without a mechanical shutter in the case where a PS-CCD sensor is used as the image sensing element, a mechanical shutter preferably is provided in order to suppress smear. However, smear cannot be completely prevented because of luminous flux reaching the PS-CCD sensor, even during the short period of time from when accumulation of the charge of the PS-CCD sensor is ended until when the PS-CCD sensor is shielded by the mechanical shutter. Various techniques have been proposed to suppress smear.
In contrast, a CMOS image sensor, which is an XY addressing type image sensing element, is advantageous in that the occurrence of smear is so small as to be negligible compared with a CCD sensor. Due to advances in enlarging technology, CMOS image sensors are often used in single reflex digital cameras, which require a large image sensing element with which high quality image data is readily obtained.
However, an XY addressing type CMOS image sensor uses a so-called rolling shutter, whereby charge is accumulated at a different timing for each row. With existing rolling shutters, it is neither possible to simultaneously reset the charge of all of the pixels, nor to simultaneously end the accumulation of charge (i.e., simultaneously read accumulated charge). A rolling shutter is not suited to taking moving objects as still images because of a lag of nearly one frame between the accumulation periods of the first and last rows of the scan screen. Thus, a mechanical shutter for controlling the exposure period is often used in image sensing apparatuses that use a CMOS image sensor.
Resetting the charge of each row in order to start accumulating the charge in the CMOS image sensor is started prior to the signal levels of accumulated charge in the row being read by an amount of time equal to the charge accumulation period. Note that charge can also be reset at a different speed to the scanning speed at which accumulated charge is read.
The following configuration, which takes advantage of this, is disclosed in Japanese Patent Laid-Open No. H11-41523. Charge accumulation is started after first resetting the charge of the CMOS image sensor one row at a time to match the traveling curve of the mechanical shutter, the CMOS image sensor is shielded with the mechanical shutter after a prescribed period, and the signal levels of the accumulated charge are then read one row at a time. Note that the traveling curve of a mechanical shutter shows the moving characteristics of the mechanical shutter, showing the position of the leading edge of the shutter blades relative to time. Exposure can be controlled by adjusting the interval between when charge is reset and when traveling of the mechanical shutter is started.
Since charge is thus reset one row at a time to match the traveling curve of the mechanical shutter, the difference in lengths of accumulation periods between the first and last rows of the scan screen can be improved to be about the same as when a mechanical shutter with front and rear curtains is used. Hereinafter, the shutter blades forming the front curtain will be called the “front blades”, while the shutter blades forming the rear curtain will be called the “rear blades”.
In addition to enabling smear to be suppressed when taking moving images because of the use of a CMOS image sensor, this configuration enables image capture with a high speed shutter, thereby making it possible to take still images of a moving object using a CMOS image sensor.
However, the front and rear blades of a mechanical shutter are generally driven by springs, with the holding power of an electromagnet often being used to hold the mechanical shutter in the start position. Thus, the traveling curve of the mechanical shutter is never constant. The traveling curve changes depending on a plurality of factors including the orientation of the image sensing apparatus, temperature, humidity, the drive voltage of the electromagnet holding the mechanical shutter, differences between individual mechanical shutters, and deterioration over time.
Thus, to reset charge at an appropriate timing that corresponds to the traveling of the mechanical shutter, a detection system for detecting the traveling curve of the mechanical shutter and a feedback system for controlling the timing at which charge is reset according to the detection result need to be provided. In view of this, Japanese Patent Laid-Open No. 2005-159418 discloses a configuration in which a plurality of photo-interrupters are disposed in the traveling direction of the front blades, and the traveling curve of the front blades is detected from the output of the photo-interrupters after scanning the front blades.
Another method of detecting the traveling curve of a mechanical shutter is disclosed in Japanese Patent Laid-Open No. 2002-064752. This method involves firstly resetting the charge of the image sensing element and acquiring a charge signal by reading the charge after a prescribed period has elapsed. Next, the charge of the image sensing element is again reset, and after the same prescribed period has elapsed, the mechanical shutter is scanned to shield the image sensing element, and the charge is read. The amounts of charge obtained from the two readings are compared, and the traveling curve is detected based on the result.
However, while the configuration of Japanese Patent Laid-Open No. 2005-159418 enables the timing at which charge is reset to be controlled by estimating the traveling curve of the rear blades in advance, the configuration of the shutter is enlarged by the provision of the photo-interrupters. Also, when optical sensors are disposed in a vicinity of the image sensing element, a configuration for preventing the light of the optical sensors from being incident on the light receiving surface of the image sensing element needs to be provided.
With the control method of Japanese Patent Laid-Open No. 2002-064752, on the other hand, the two charge amounts cannot be correctly compared if there is a change in luminance resulting from a change in the state of the object between the two charge accumulation periods, since several hundred milliseconds are required to read the charge of all of the pixels in the image sensing element. In such cases, the traveling curve of the mechanical shutter cannot be precisely detected. Further, if detection of the traveling curve of the mechanical shutter is started immediately before a normal image sensing operation, it takes time until the shot is actually taken since the image sensing operation is designated, resulting in an image sensing apparatus with a significant release time lag that is not convenient to use.
The time needed to detect the traveling curve can be shortened by reading the charge of a reduced number of pixels rather than all of the pixels in the image sensing element in order to detect the traveling curve of the mechanical shutter. However, a precise traveling curve cannot be derived in this case if the intervals between pixels whose charge is read out are long.